Pull tube stress joint for offshore platform

ABSTRACT

The present disclosure provides a system and method for supporting a catenary riser coupled to an offshore platform system including a pull tube and a pull tube stress joint for girth weld stress reduction and improved fatigue performance. A pull tube sleeve is coupled around a welded connection of the pull tube. The sleeve has a larger inside diameter than an outer diameter of the pull tube, and a hardenable fill material is filled into the space between the sleeve and the pull tube so that there is no annular gap between the sleeve and the pull tube. The fill material provides a supportive coupling between the sleeve and the pull tube. The sleeve, the pull tube, or both can also have one or more gripping surfaces formed in or on their surfaces to retain the fill material within the space between the sleeve and the pull tube.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure generally relates to the production of hydrocarbons from subsea formations. More particularly, the disclosure relates to the risers and related support structures used in such production.

2. Description of the Related Art

In producing hydrocarbons from subsea formations, a number of wells are typically drilled into the sea floor in positions that are not directly below or substantially within the outline of an offshore floating platform, such as a floating offshore production platform. The produced hydrocarbons are subsequently exported via subsea pipelines or other means. Current engineering practice links the offset wells with the offshore platform through risers that have a catenary curve between the platform and the sea floor. Wave motion, water currents, and wind cause movement of the floating offshore structure and/or risers themselves with corresponding flex and stress in the risers. The current state of the art has accommodated the flex in the risers by incorporating flexible joints at suitable locations between pipe segments in the riser. However, the flexible joints are more expensive and less reliable than pipe segments that are welded together.

Steel Catenary Risers (SCRs) are designed to be coupled to the floating offshore structure through pull tubes extending from the lower keel of the offshore structure to the upper part of the offshore structure. A pull tube is generally a long conduit that forms a guide through which the SCR is pulled from the seafloor and coupled to the offshore structure. The pull tube is attached to the offshore structure at an angle from the vertical so as to be in line with a natural catenary angle that the installed SCR would assume on a calm day. As the offshore structure shifts laterally and vertically, the pull tube helps reduce stresses on the SCR. However, the pull tube itself is then stressed and can fail with time. The pull tube is attached to the offshore structure at one or more attachment points and thus flexes about its attachment points to the offshore structure as the SCR flexes and bends from the movement of the floating offshore structure. A first attachment point can be located a distance from the lower end of the pull tube. A second attachment point for the pull tube to the offshore structure can be at a distance further upward from the first attachment point to allow additional flexibility in the pull tube. Further, the pull tube can be provided with a bending stiffness that varies from the first attachment point to the lower end of the pull tube. Typically, a tapered stress joint is placed along the pull tube adjacent one of the attachment points and is sized to control the SCR stress.

There are two types of stress joints that have been used in the past. The first one is an assembly of pipe segments welded together. The pipe segments typically have a progressively smaller wall thickness for each segment of a given inside diameter that results in a tapered assembly of the segments with the thinnest segment distal from the middle of the welded assembly to allow more flexibility at the end of the assembly for the SCR. Such assemblies typically are challenged by fatigue performance at the welds between the segments for the many years in which the SCR will likely be used. The second type of stress joint is a forged tapered stress joint. The forging accomplishes a similar goal as the first type by progressively thinning the wall thickness toward the end of the forging typically in the length of 40 ft. However, due to the desired length of a pull tube stress joint, additional pull tube segments are typically welded to the forging. Thus, the challenge is still fatigue performance at the welds between the segments and forging. Another challenge can be cost and manufacturing schedules specific to a lengthy forging piece.

More particularly, FIG. 1 is an exemplary prior art schematic of a pull tube stress joint. The pull tube stress joint 50 is adapted to allow a riser 53 to be pulled therethrough and includes a tapered middle section 51, which can be one of the two types described above of a progressively smaller wall thickness of an assembly of pipe segments or a continuous forging. The middle section 51 has a length “L”, which can for example be about 40 feet (12 meters) and is typically centrally disposed relative to a pivot point “A”, so that a ½ L length extends 20 feet (6 meters) outward therefrom in this example. A pull tube joint 52 is welded to the end of the middle section 51 at welding B about 20 feet (6 meters) from the pivot point A. The stresses at welding B are such that special and expensive welding procedures known as a “C Class Girth Weld” are typically specified to attempt to reduce fatigue at the welding B at the 20 foot (6 meter) location from the pivot point A. While a longer middle section could be used to extend the ½ L length from the pivot point A, the expense and timing of production and handling make such an option unsuitable for practical reasons.

An improvement to the pull tube stress joint of FIG. 1 is shown in US Publ. No. 2011/0048729. The shown pull tube sleeve stress joint includes at least one sleeve surrounding a length of the pull tube with an annular gap between the sleeve and pull tube and a link ring therebetween. For embodiments having a plurality of sleeves, a first sleeve can be spaced by an annular first gap from the pull tube and coupled thereto with a first ring between the pull tube and the first sleeve, and a second sleeve can be spaced by an annular second gap from the first sleeve and coupled thereto with a second ring between the first sleeve and the second sleeve.

Despite this improvement, there remains then a need to simplify the structure of a pull tube stress joint system for catenary risers and yet still provide for a suitably long lasting, cost effective pull tube stress joint.

BRIEF SUMMARY OF THE INVENTION

The present disclosure provides an improved design for a system and method for supporting a catenary riser from an offshore platform that includes a pull tube stress joint and associated pull tube. The new design efficiently results in a pull tube stress joint sleeve coupled to a pull tube at a welded connection of the pull tube, the sleeve having a larger inside diameter than an outer diameter of the pull tube at the welded connection, and a hardenable fill material filled into the space between the sleeve and the pull tube so that there is no annular space between the sleeve and the pull tube. Without limitation, the fill material can be concrete, grout, or other cement-based materials; rubberized materials, including rubberized grout; polymeric materials, such as epoxies and phenolics; and other materials that can be filled into the space between the sleeve and the pull tube to provide a supportive coupling between the sleeve and the pull tube. The sleeve, the pull tube, or both can also have one or more gripping surfaces formed in or on their surfaces, such as ribs, indentions, projections, or other surface irregularities above or below the nominal surface of the sleeve and/or pull tube. With the sleeves, the stress at the girth welds can be significantly reduced, and then the fatigue performance of the entire pull tube stress assembly will be significantly improved.

The disclosure provides a system for supporting a catenary riser coupled to an offshore platform, comprising: a pull tube having an outer diameter surface and an inner diameter surface, the inner diameter surface being sized to allow the riser to pass therethrough, the pull tube having a lower end disposed downward from the offshore platform and at an upper portion distal from the lower end disposed toward the offshore platform, and the pull tube further having one or more segments welded together to establish one or more welded connections with the pull tube extending longitudinally on both sides of the welded connections; a pull tube guide coupled to the offshore platform and coupled to the outer diameter surface of the pull tube between the lower end and the upper portion; a pull tube stress joint first sleeve disposed around a length of the pull tube at a first welded connection and longitudinally extending on both sides of the first welded connection, the first sleeve having an outer diameter surface and an inner diameter surface, the sleeve inner diameter surface being larger than the pull tube outer diameter surface; and a first quantity of fill material coupled between the sleeve inner diameter surface and the pull tube outer diameter surface to fill a cross section of the annular gap between the two surfaces.

The disclosure also provides a method of supporting a catenary riser coupled to an offshore platform, comprising: providing a plurality of segments of a pull tube having an outer diameter surface and an inner diameter surface, the inner diameter surface being sized to allow the riser to pass therethrough; welding at least two of the segments together to establish one or more welded connections with the pull tube extending longitudinally on both sides of the welded connection; coupling the pull tube to the offshore platform between a lower end of the pull tube disposed downward from the offshore platform and at an upper portion of the pull tube distal from the lower end disposed toward the offshore platform; coupling a pull tube stress joint first sleeve around a first welded connection of the pull tube, the first sleeve having an outer diameter surface and an inner diameter surface, the sleeve inner diameter surface being larger than the pull tube outer diameter surface; and filling a gap between the sleeve inner diameter surface and the pull tube outer diameter surface with a first quantity of a fill material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an exemplary prior art schematic of a pull tube stress joint.

FIG. 2 is a side view schematic diagram illustrating an exemplary system for supporting a catenary riser coupled to an offshore platform with a pull tube, a pull tube guide coupled to the platform and supporting the pull tube, and a plurality of pull tube stress joint sleeves at locations along the pull tube.

FIG. 3 is a side view schematic diagram illustrating the pull tube with the pull tube stress joint sleeves.

FIG. 4 is a side cross-sectional view schematic diagram illustrating the pull tube, pull tube guide, and pull tube stress joint sleeves.

FIG. 5 is a detail side cross-sectional view schematic diagram illustrating an exemplary embodiment of a pull tube stress joint assembly of FIG. 4.

DETAILED DESCRIPTION

The Figures described above and the written description of specific structures and functions below are not presented to limit the scope of what Applicant has invented or the scope of the appended claims. Rather, the Figures and written description are provided to teach any person skilled in the art to make and use the inventions for which patent protection is sought. Those skilled in the art will appreciate that not all features of a commercial embodiment of the inventions are described or shown for the sake of clarity and understanding. Persons of skill in this art will also appreciate that the development of an actual commercial embodiment incorporating aspects of the present disclosure will require numerous implementation-specific decisions to achieve the developer's ultimate goal for the commercial embodiment. Such implementation-specific decisions may include, and likely are not limited to, compliance with system-related, business-related, government-related and other constraints, which may vary by specific implementation, location and from time to time. While a developer's efforts might be complex and time-consuming in an absolute sense, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in this art having benefit of this disclosure. It must be understood that the inventions disclosed and taught herein are susceptible to numerous and various modifications and alternative forms. The use of a singular term, such as, but not limited to, “a,” is not intended as limiting of the number of items. Also, the use of relational terms, such as, but not limited to, “top,” “bottom,” “left,” “right,” “upper,” “lower,” “down,” “up,” “side,” and the like are used in the written description for clarity in specific reference to the Figures and are not intended to limit the scope of the invention or the appended claims. Where appropriate, some elements have been labeled with an “A” or “B” to designate various members of a given class of an element. When referring generally to such elements, the number without the letter is used. Further, such designations do not limit the number of members that can be used for that function.

In general, the present disclosure provides an improved design for a system and method for supporting a catenary riser from an offshore platform that includes a pull tube stress joint and associated pull tube. The new design efficiently results in a pull tube stress joint sleeve coupled to a pull tube at a welded connection of the pull tube, the sleeve having a larger inside diameter than an outer diameter of the pull tube at the welded connection, and a hardenable fill material filled into the space between the sleeve and the pull tube so that there is no annular space between the sleeve and the pull tube. Without limitation, the fill material can be concrete, grout, or other cement-based materials; rubberized materials, including rubberized grout; polymeric materials, such as epoxies and phenolics; and other materials that can filled into the space between the sleeve and the pull tube to provide a supportive coupling between the sleeve and the pull tube. The sleeve, the pull tube, or both can also have one or more gripping surfaces formed in or on their surfaces, such as ribs, indentions, projections, or other surface irregularities above or below the nominal surface of the sleeve and/or pull tube.

FIG. 2 is a side view schematic diagram illustrating an exemplary system for supporting a catenary riser coupled to an offshore platform with a pull tube, a pull tube guide coupled to the platform and supporting the pull tube, and a plurality of pull tube stress joint sleeves at locations along the pull tube. The pull tube 1 is coupled to the offshore platform 14, such as with an upper support 2, generally at an upper portion 3A of the pull tube. A lower end 3B of the pull tube 1 is generally directed downward from the offshore platform 14 toward a seafloor 54 and the end is flared open to insert and guide a riser 4, such as a Steel Catenary Riser (SCR), from the seafloor into the pull tube 1. The pull tube 1 is maintained in proximity to the offshore platform 14, such as in proximity to a soft tank 5, by a pull tube guide 6, also referenced as a “casting guide”. The pull tube guide 6 is coupled to the outer diameter surface of the pull tube 1 between the lower end 3B and the upper portion 3A. The pull tube guide 6 is coupled to the offshore platform 14 and extends laterally outward from the platform to provide a transition of angle of the catenary shape of the riser 4, as the riser approaches the offshore platform 14. One or more pull tube stress joint sleeves 7A, 7B, 7C, and 7D (and others as appropriate) surround one or more portions of the pull tube 1 generally where a welded connection is made between segments of the pull tube, as described below.

FIG. 3 is a side view schematic diagram illustrating the pull tube with the pull tube stress joint sleeves. FIG. 4 is a side cross-sectional view schematic diagram illustrating the pull tube, pull tube guide, and pull tube stress joint sleeves. FIG. 5 is a detail side cross-sectional view schematic diagram illustrating an exemplary embodiment of a pull tube stress joint assembly of FIG. 4. The figures will be described in conjunction with each other. Multiple segments, such as segments 8, 9, 10, 11, and 12, form the pull tube 1. The segments are welded together to form welded connections, such as welded connections 15 and 16, between the segments, where the pull tube 1 extends longitudinally both directions from the welded connections. Some segments, such as segment 9, can have different wall thicknesses to provide additional strength in high stress portions of the pull tube. While the pull tube 1 itself may be able to withstand bending stresses as the catenary riser 4 moves back and forth within the pull tube, the welded connections without special precautions closest to the pull tube guide 6 incur higher stresses and may fatigue and fail. Typically, expensive Class C welds are required for these welded connections as explained in the above background section. Without limitations, exemplary lengths of segments are shown as 40 feet (12 meters), and other lengths are possible.

However, the present invention allows use of more standard welds. In at least one embodiment, a portion of the segment 9 with the thickest wall in close proximity to the guide 6 is not welded and thus no welded connection is subject to the full stress of the bending of the pull tube 1 in the guide 6 as a focal point of the bending stress. At the ends of the segment 9, the segments 8 and 10 can be welded to form welded connections 15, 16. As the pull tube extends further away from the guide 6, the stresses lessen on the pull tube and welded connections of further segments of the pull tube may not be sufficiently stressed to warrant the use of a sleeve 7 around such further welded connections.

One or more pull tube stress joint sleeves 7A, 7B can be coupled to the pull tube 1 at the welded connections 15, 16. The sleeves 7 are disposed around a length of the pull tube at the welded connections. The sleeve extends longitudinally on both sides of the welded connection. The sleeves have an outer diameter surface and an inner diameter surface, where the sleeve inner diameter surface is larger than the pull tube outer diameter surface and forms a space 17 therebetween that is filled as explained herein. While the number of sleeves can vary from one to several, it is envisioned that generally a sleeve can be advantageously used at each of the nearest welded connections along the length of the pull tube as the pull tube extends from the guide 6.

A quantity of hardened fill material 13 is coupled between the inner diameter surface of the sleeve 7 and the outer diameter surface of the pull tube 1 to fill a cross section of the annular space between the two surfaces. Without limitation, the fill material can be concrete, grout, or other cement-based materials; rubberized materials, including rubberized grout; polymeric materials, such as epoxies and phenolics; and other materials that can filled into the space between the sleeve and the pull tube to provide a supportive coupling between the sleeve and the pull tube. The purpose of the fill material is to transfer the bending load of the pull tube near the welded connection to the sleeve surrounding the pull tube. Thus, a hard fill material is envisioned rather than a pliable and flexible material.

In at least one embodiment, the fill material can initially be a fluid that can be poured or injected into the space 17 and then hardened to function as described. One or more annular caps 18A, 18B can be positioned in the space such as at the ends of the sleeve 7 to retain the fluid fill material at least until the fill material can sufficient harden. A port 22 can be formed in the sleeve 7, the cap 18, or other appropriate location to facilitate filling of the space 17. A line 24 can be coupled from the port 22 to a tank 26 of a flowable fill material 28. A pump (not shown) can be used to transfer the fill material from the tank 26 to the space 17. In general, it is advantageous to fill the entire space 17 with the fill material to be able to transfer a full load from the pull tube into the sleeve to diffuse the stress on the pull tube. However, some portion of the space between the sleeve 7 and the pull tube 1 may not have a complete filling and the term “fill” or “filling” and the like herein is not restricted to a complete filling of every portion of the space 17 by the fill material 13, but is meant to include filling of the space across at least one cross section between the sleeve and the pull tube.

The sleeve 7, the pull tube 1, or both can also have one or more gripping surfaces 20 formed in or on their surfaces, such as indentions 20A, ribs and projections 20B, or other surface irregularities above or below the nominal surface of the sleeve and/or pull tube. The gripping surfaces assist in restraining the fill material in position between the sleeve and pull tube and restraining the sleeve relative to the pull tube.

Other and further embodiments utilizing one or more aspects of the inventions described above can be devised without departing from the spirit of the disclosed invention. For example and without limitation, the pull tubes, sleeves, and components thereof, can be round or other geometric shapes, so that the use of the term “diameter” is to be construed broadly to mean an inside or outside periphery, as the case may be, that may or may not be round. The embodiments have generally been described in terms of welding, because the general state of the art is conducive to welding, but the invention is not limited to welding and can include any suitable form of coupling, such as clamping, grouting, fastening, and other coupling means as further defined below. Further, the use of a sleeve as a stress joint around the pull tube within the pull tube guide is contemplated and can be in addition to the pull tube stress joint sleeves around the welded connection described herein.

Further, the various methods and embodiments of the system can be included in combination with each other to produce variations of the disclosed methods and embodiments. Discussion of singular elements can include plural elements and vice-versa. References to at least one item followed by a reference to the item may include one or more items. Also, various aspects of the embodiments could be used in conjunction with each other to accomplish the understood goals of the disclosure. Unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising,” should be understood to imply the inclusion of at least the stated element or step or group of elements or steps or equivalents thereof, and not the exclusion of a greater numerical quantity or any other element or step or group of elements or steps or equivalents thereof. The device or system may be used in a number of directions and orientations. The term “coupled,” “coupling,” “coupler,” and like terms are used broadly herein and may include any method or device for securing, binding, bonding, fastening, attaching, joining, inserting therein, forming thereon or therein, communicating, or otherwise associating, for example, mechanically, magnetically, electrically, chemically, operably, directly or indirectly with intermediate elements, one or more pieces of members together and may further include without limitation integrally forming one functional member with another in a unity fashion. The coupling may occur in any direction, including rotationally.

The order of steps can occur in a variety of sequences unless otherwise specifically limited. The various steps described herein can be combined with other steps, interlineated with the stated steps, and/or split into multiple steps. Similarly, elements have been described functionally and can be embodied as separate components or can be combined into components having multiple functions.

The inventions have been described in the context of preferred and other embodiments and not every embodiment of the invention has been described. Obvious modifications and alterations to the described embodiments are available to those of ordinary skill in the art. The disclosed and undisclosed embodiments are not intended to limit or restrict the scope or applicability of the invention conceived of by the Applicant, but rather, in conformity with the patent laws, Applicant intends to protect fully all such modifications and improvements that come within the scope or range of equivalent of the following claims. 

What is claimed is:
 1. A system for supporting a catenary riser coupled to an offshore platform, comprising: a pull tube having an outer diameter surface and an inner diameter surface, the inner diameter surface being sized to allow the riser to pass therethrough; the pull tube having a lower end disposed downward from the offshore platform and at an upper portion distal from the lower end disposed toward the offshore platform; and the pull tube further having one or more segments welded together to establish one or more welded connections with the pull tube extending longitudinally on both sides of the welded connections; a pull tube guide coupled to the offshore platform and coupled to the outer diameter surface of the pull tube between the lower end and the upper portion; a pull tube stress joint first sleeve disposed around a length of the pull tube at a first welded connection and longitudinally extending on both sides of the first welded connection, the first sleeve having an outer diameter surface and an inner diameter surface, the sleeve inner diameter surface being larger than the pull tube outer diameter surface; and a first quantity of fill material coupled between the sleeve inner diameter surface and the pull tube outer diameter surface to fill a cross section of the annular gap between the two surfaces.
 2. The system of claim 1, wherein the first sleeve, the pull tube at the first welded connection, or a combination thereof have one or more gripping surfaces configured to provide displacement resistance to the fill material.
 3. The system of claim 1, further comprising an annular cap disposed between the first sleeve inner diameter surface and the pull tube outer diameter surface and configured to retain the fill material in position between the sleeve and the pull tube until the fill material is hardened.
 4. The system of clam 1, further comprising an inlet port in the first sleeve configured to allow the fill material to be injected into the space between the first sleeve and the pull tube.
 5. The system of claim 1, further comprising a pull tube stress joint second sleeve disposed around a second welded connection distal from the first welded connection and having a second quantity of the fill material between the second sleeve and the pull tube at the second welded connection.
 6. The system of claim 1, wherein the fill material comprises cement, polymeric material, rubber, or a combination thereof.
 7. The system of claim 1, wherein at least one of the welded connections is nearest to the pull tube guide along the pull tube.
 8. The system of claim 1, wherein the upper portion is coupled to the offshore platform distal from the pull tube guide.
 9. The system of claim 1, wherein one or more of the segments of the pull tube have a different wall thickness along the length of the segment.
 10. A method of supporting a catenary riser coupled to an offshore platform, comprising: providing a plurality of segments of a pull tube having an outer diameter surface and an inner diameter surface, the inner diameter surface being sized to allow the riser to pass therethrough; welding at least two of the segments together to establish one or more welded connections with the pull tube extending longitudinally on both sides of the welded connection; coupling the pull tube to the offshore platform between a lower end of the pull tube disposed downward from the offshore platform and at an upper portion of the pull tube distal from the lower end disposed toward the offshore platform; coupling a pull tube stress joint first sleeve around a first welded connection of the pull tube, the first sleeve having an outer diameter surface and an inner diameter surface, the sleeve inner diameter surface being larger than the pull tube outer diameter surface; and filling a gap between the sleeve inner diameter surface and the pull tube outer diameter surface with a first quantity of a fill material.
 11. The method of claim 10, further comprising forming one or more gripping surfaces on the first sleeve, the pull tube at the first welded connection, or a combination thereof to provide displacement resistance to the fill material.
 12. The method of claim 10, further comprising capping an annular space between the first sleeve inner diameter surface and the pull tube outer diameter surface; retaining the fill material in position between the sleeve and the pull tube; and allowing the fill material to hardened while retaining the fill material.
 13. The method of clam 10, wherein filling the gap between the sleeve inner diameter surface and the pull tube outer diameter surface further comprises injecting flowable fill material through an inlet port in the first sleeve.
 14. The method of claim 10, further comprising coupling a pull tube stress joint second sleeve around a second welded connection of the pull tube, the second sleeve having an outer diameter surface and an inner diameter surface, the sleeve inner diameter surface being larger than the pull tube outer diameter surface; filling a gap between the sleeve inner diameter surface of the second sleeve and the pull tube outer diameter surface with a second quantity of fill material.
 15. The method of claim 10, wherein coupling the pull tube to the offshore platform comprises coupling the pull tube to a pull tube guide that is coupled to the offshore platform.
 16. The method of claim 15, further comprising coupling the upper portion of the pull tube to the offshore platform distal from the pull tube guide. 